Redispersible polymer powders

ABSTRACT

The invention concerns redispersible polymer powders which can be used in a variety of ways, for example, in hydraulically binding systems, but also as binder in greatly varied applications and which can be produced by spray-drying of aqueous dispersions. The sprayable aqueous dispersions are prepared by mixing an aqueous polymer dispersion 1 (containing polymer I) with an aqueous polymer dispersion 2 (containing polymer II) where the diameters of the particles of polymer I are greater by a factor of 5 to 150, preferably 15 to 50, than the diameter of the particles of polymer II, and polymer I is a polymer which forms films at room temperature, while polymer II has a glass transition temperature which is higher than that of polymer I, is insoluble in water and thus it is present in the undissolved form in the aqueous dispersion to be sprayed. The polymer powders according to the invention excel by a clearly improved resistance to blocking and very good storage stability.

The present invention concerns redispersible polymer powders, whichexcel especially by good storage stability and can be used in many ways,for example, in hydraulically binding systems, but also as a binder inthe most varied applications, and can be prepared by spray-drying ofaqueous dispersions.

The use of polymer powders as additive, especially for hydraulicallybinding systems or as binder in coating compositions is generally known.Such additives improve the application properties of the systems inwhich they are used. Thus, when used in hydraulically bindingcompositions, the bending and tensile strength of construction materialsto which they are added, are improved. Moreover, they have an improvinginfluence on the processability of the construction materials.

The production of such powders is known and, for example, is describedin the following patents:

In EP 0134451, the use of various naturally occurring water-solublesubstances, such as starches, starch derivatives, proteins and/orwater-soluble polymers are named as spray additives in the preparationof redispersible dispersion powders.

DE-OS 4021216 describes the preparation of a polymer powder, where afilm-forming dispersion polymer is sprayed with an completely orpartially neutralized water-soluble copolymer consisting of 50 to 80weight % of unsaturated carboxylic acids and an alkene and/or styrene asspray additive. According to EP 0 671 435, aqueousprotective-colloid-containing acrylate dispersions can be sprayed topowder redispersible binders using 0.01 to 30 weight % of water-solubleor water-dispersible resins, which consist of 30 to 50 weight % ofhydrophilic, water-soluble and salt-forming monomers and 50-70 weight %of hydrophobic, essentially water-insoluble monomers. In EP 0522791, thepreparation of powders by spraying of core-shell products is described,where an alkali-insoluble core is surrounded by an alkali-soluble shell.The core-shell dispersion is sprayed after neutralization with an amineor a base.

All the redispersible powders named above are based on a uniform basicprinciple of preparation: To the polymer to be sprayed, a spraying aidis added, which is first in the dissolved form at the beginning of thespray-drying process and deposits on the particles to be sprayed duringdrying, as a “protective shield”. The spraying aid has a great influenceon the properties of the powder. First of all, it should provide as gooda redispersibility to the polymer powder as possible, and, on the otherhand, together with the commonly used antiblocking agent, should preventblocking of the powder during storage. Moreover, the spraying aid shouldnot have an adverse influence on the application-technologicalproperties of the polymer to be sprayed, but should influence these asadvantageously as possible.

The task of the present invention was to prepare a redispersible polymerpowder which is characterized, in contrast to the above, by the factthat, with the same amount of spraying aid (weight %, based on thepolymer to be sprayed), a clearly improved resistance to blocking isachieved. Moreover, the preparation of the powder according to theinvention should result in a simplification of the manufacturingtechnology.

In developing the solution, it was shown that good redispersibledispersion powders can be prepared not only in the usual way, by the useof a spraying aid present in the dissolved form, but that, surprisingly,this is possible even when using an additive that is not dissolved, whenthis has a particle size which is clearly smaller than the particle sizeof the dispersion to be sprayed.

This is achieved by redispersible polymer powders, prepared byspray-drying of an aqueous dispersion, which is obtained by mixing anaqueous polymer dispersion 1 (containing polymer I) with an aqueouspolymer dispersion 2 (containing polymer II), where the diameter of theparticles of polymer I is greater than the diameter of the particles ofpolymer H by a factor of 5 to 150, preferably 10 to 50, and polymer I isa polymer that forms films at room temperature, while polymer II has aglass-transition temperature which is higher than that of polymer I, isinsoluble in water and thus it is present in the undissolved form in theaqueous dispersion to be sprayed.

The glass temperature of polymer H should be higher than the temperatureat the exit of the spray dryer. The amount of polymer II, which isneeded (weight % based on polymer I), in order to achieve a certainresistance to blocking, which becomes smaller as polymer I and polymerII differ more with regard to their particle size. The amount of polymerII which is used lies at 5 to 40 weight %, preferably 10 to 30 weight %.Before spraying the additive, surface-active substances can be added tothe aqueous dispersion to improve the redispersibility.

Preferably, polymer II consists of 5 to 80 weight %, especially 20 to 50weight % of a copolymerized, unsaturated carboxylic acid and is thusentirely or partially soluble in alkali, while the pH value of theaqueous dispersion to be sprayed is kept so low that polymer II ispresent in the undissolved form.

Advantageously, methacrylic acid or acrylic acid is used as theunsaturated carboxylic acid.

The spray drying of the aqueous dispersion is done in a commercial pilotplant spray dryer in cocurrent. Air is used as drying gas, which entersthe dryer at a temperature of 115EC and leaves it at a temperature of60EC. The dispersions I and II are always sprayed in as a mixture ofdispersion through a two-material nozzle. The addition of antiblockingagents (ABM) is done through a second two-material nozzle also incocurrent. The powder is deposited in a cyclone which is connected atthe end.

In the examples given below, polymer dispersions with the followingcomposition were used:

Particle size Dispersion (TGv nm) A styrene/butyl acrylate/acrylamide600 (Tg according to Fox: 0EC) B styrene/butadiene 200 (Tg according toFox: 5EC) C methyl methacrylate/methacrylic acid 25 (35 weight %methacrylic acid) D methyl methacrylate/methacrylic acid 260 (35 weight% methacrylic acid) E styrene 20 F styrene 190

In Table 1, the Examples are characterized more closely and essentialdata are summarized about the properties of the polymer powdersproduced.

TABLE 1 polymer powder sedim. sedim. 1/24 aqueous dispersion 1/24 h hABM blocking Example pH polymer I % polymer II % (pH < 7) (pH > 10) (%)resistance 1 <7 A 80 C 20 4/24 1/7  5 12 2 <7 A 85 C 15 6/27 2/10 5 17 3V <7 A 80 D 20 9/38 7/27 5 30 4 V >10 A 80 C 20 2/8  5 36 5 >7 A 80 E 205/17 5 24 6 V >7 A 80 F 20 25/46  5 60 7 >7 B 80 E 20 4/14 5 20 8 V >7 B80 F 20 15/35  5 50 V = Comparison Example

The addition of other additives (for example, surface-active substances)was done so that samples compared with one another always contained thesame amount of them.

It can be seen in Table I that the ratio of the particle sizes ofpolymer I: polymer II has a decisive influence on the sedimentation andtendency to blocking (comparison of Example 1 with Example 3, comparisonof Example 5 with Example 6 and comparison of Example 7 with Example 8).It can be seen that the examples prepared according to the inventionshow clearly better values, both with regard to sedimentation as well asresistance to blocking (see in this connection “Explanation of themethods of investigation”).

Example 1 differs from Comparison Example 4 only in the fact thatExample 4 was used at a pH >10 and thus polymer II was sprayed in thedissolved state. It was found that this made the resistance to blockingclearly worse.

The polymer powders from Example I and from Example 4 were added to amixture of 78 weight % of standard sand and 22 weight % of cement (32.5)and a mortar was prepared from this (plastic : cement ratio always 0.1,water: cement 0.45), which was then investigated according to EN 196.

It can be seen from Table 2 that the greatly improved resistance toblocking of the powder produced according to the invention is notreached at the expense of significant application-technologicalproperties. Both the bending tensile strength, as well as thecompressive strength of the produced concrete show better values whenprepared with the addition of the powder according to Example 1.

TABLE 2 bending tensile extent of density strength expansion prisms(N/mm²), compressive strength Example (mm) (g/cm³) 14 days (N/mm²), 14days 1 148 2.11 7.4 37 4 V 152 2.12 6.9 33

Explanation of the Methods of Investigation:

Blocking test: First, a tablet is formed from the powder and a weight ismade to act on a stamp, which is loaded onto the powder in a metalcylinder of 50 mm diameter for 24 hours. A load is chosen which isgreater than occurs approximately during storage in pallets, in order tomake tabletting possible, even in the case of powders which show hardlyany tendency to blocking, so that this method can also be used in thecomparison with powders having very good blocking behavior.

The obtained tablet is introduced in the horizontal position into ahollow cylinder rotating along its longitudinal axis, the surface ofwhich is covered with sieving fabric. The cylinder turns at a velocityof 40 rpm, and the weight of the powder falling through the sievingfabric is determined. The time is determined within which 50% of theweight of the tablet went through the sieve. The shorter this time, thelower the tendency of the powder to blocking. Under the selectedconditions, commercial powders reach values of 10 to 20 seconds.

Sedimentation: This value yields information about the redispersibilityof the investigated powder.

For this purpose, the powder is redispersed in water at a ratio of 1:1and then diluted, 50 g of the 5% redispersions are filled intocalibrated tubes and after 1 hour and 24 hours, the height of thesedimented layer is measured; the lower the obtained values the betteris the redispersibility of the powder.

In Table 1, in the case of using alkali-soluble polymers, in addition,the sedimentation of the redispersion prepared at pH>10 is determined,since this comes closer to the conditions when using the powder in themortar.

Under the selected redispersion conditions, commercial powders showvalues of approximately 2 and 8-10 mm.

What is claimed is:
 1. Redispersible polymer powder prepared byspray-drying of an aqueous dispersion, wherein this dispersion isobtained by mixing an aqueous polymer dispersion I (containing polymerI) with an aqueous polymer dispersion 2 (containing polymer II) wherethe diameters of the particles of polymer I are greater by a factor of 5to 150 than the diameters of the particles of polymer II and polymer Iis a polymer which forms a film at room temperature, while polymer IIhas a glass transition temperature higher than polymer I, is insolublein water and thus is present in the undissolved form in the aqueousdispersion to be sprayed and is used in an amount of 5 to 40 weight %,based on the total weight of polymer I and polymer II.
 2. Redispersiblepolymer powder according to claim 1, wherein polymer II contains 5 to 80weight %, preferably 20 to 50 weight % of copolymerized unsaturatedcarboxylic acids, so that it is entirely or partially soluble in alkali,but the pH value of the aqueous dispersion to be sprayed is kept so lowthat polymer II is present in the water-insoluble form and thatmethacrylic acid or acrylic acid are used as the unsaturated carboxylicacids.
 3. Redispersible polymer powder according to claims 1 and 2,wherein the diameter of the particles of polymer I are greater by afactor of 10 to 50 than the diameters of the particles of polymer II. 4.Redispersible polymer powder according to claims 1 to 3, wherein thepolymer II is present in a weight of 10 to 30 weight % based on thetotal weight of polymer I and polymer II.